Rotary piston for a roots blower



Oct. 1, 1963 c. HUBRICH 3,105,634"

ROTARY PISTON FOR A ROOTS BLOWER Filed Dec. 14. 1961 2 Sheets-Sheet 1 i-*Piston diameter Fig. 4

6 l 65 INVENTOR Cbrvs/a/efi Hob/v2 ilnited states 3,105,634 RQTARY PHSTON FGR A ROQTS BLOWER Christoph Hubrich, ()fienbach (Maim-Bieber, Germany, assignor to Poiysius G.rn.h.H., Neuhechum, Germany Filed Dec. 14, 196i, Ser. No. 159,306 Claims priority, application Germany Dec. 27, 19 50 4 Claims. (Cl. 230-141) The present invention relates to a rotary piston for a Roots blower. In certain fields in which an absolutely oil free conveyance, erg. with conveyors for food or sensitive chemical substances of high purity, and constancy or stability in the volume is required over a relatively wide pressure range (conveyance of gas), or where rough operations occur and a long life of the respective installation is involved, the advantages of the Roots blower are so predominant that in spite of its relatively high purchase price, it is indispensable :for modern industry and has become the matter of fact component of large as well as smaller plants. The only drawback inherent heretofore to a Roots blower consists in that the rotary pistons are bound to a certain shape determined by gear technical conditions, which shape, due to the required precision, is rather complicated and dilficult to meet from a production point and thus considerably increases the cost of the entire blower.

Therefore, numerous efforts have been made to develop automatic production methods for obtaining the desired shape and thereby to eliminate the only weak point in the construction of Roots blowers. Thus, a production machine has been developed according to the German periodical Automobiltechnische Zeitschrift, No. 1, 1943, vol. 46, which by means of cams is intended to facilitate the machining or, more specific-ally, the turning of the blade while the carriage carrying the turning tool is reciprocated by a profiled slide track, whereas a second slide track controls the cutting angle of the turning tool. This method, however, has never gone beyond the test stage because the accumulation of too many sources of error, such as lack of precision in the slide track, air in the many bearings, etc., was unable to yield the precision indispensable with this machine element.

A second suggestion described in the German periodical Industrie Rundschau, 1948, No. 10, was able to replace the profiled sliding tracks by coupling cam transmissions, but this machine had its guiding transmission provided with so many bearings that it too failed in practice for the above mentioned reasons.

Attempts have furthermore been made to produce the rotary piston as welding construction from stamped and pre-pressed or otherwise shaped metal parts with corresponding supports. These heretofore known construc tions, however, likewise did not meet with success in practice because no welding construction can ever yield the required precision and furthermore there is no assurance as to the obtainment of the desired profile shape because such precision can be obtained only by machining the respective parts to the desired precision.

Therefore, it was suggested to subdivide the blade into a component with a cylindrical surface so that the newly created separating surface can be produced on lathes or grinding machines with the desired precision. The second component was so designed that the profile limitation of the teeth or piston heads followed circles and thus could likewise be precisely manufactured on lathes. However, there remain the unsolved problem and difiiculty of the machining of the inwardly arched center portion. While it was suggested to produce this last mentioned intermediate member by drilling or milling, it will easily be realized that the intermediate portion of the blade for reasons inherent to the production of the teeth cannot have a uniform circular circumferential line. While 3,105,634; Patented Oct. 1, 1963 it might suggest itself to carry out such machining by means of a profiled cutter which, however, for obvious reasons is diflicult to produce inasmuch as it has no circular contour, such machining, however, would, of course, have the same drawbacks with regard to the turning operation as mentioned above.

It is, therefore, an object of the present invention to provide a rotary piston for Roots blowers, which will overcome the above mentioned drawbacks.

It is another object of this invention to provide a rotary piston for a Roots blower which can be produced by a simple turning operation without requiring special devices or the like.

These and other objects and advantages of the invention will appear more clearly from the following specifica tion in connection with the accompanying drawings, in which:

FIG. 1 diagrammatically illustrates one of the two possible profiles of Roots blades which profiles as to their entire development can be produced from only two or a maximum of three circular arcs, the center points of which are located only on the longitudinal axis of the rotary piston profile.

FIG. 2 shows a Roots blade shape according to FIG. 1 composed of three parts.

FIG. 3 diagrammatically illustrates the other of the two possible Roots blade profile shapes which as to their entire development can be produced from only two or a maximum of three circular arcs the center points of which are located on the two main axes of the rotary piston profile.

FIG. 4 shows the Roots blade profile shape according to FIG. 3 composed of three parts.

FIG. 5 diagrammatically illustrates the Roots blade profile shape according to FIG. 3 with other subdivisions.

FIG. 6 shows a Roots blade profile shape according to FIG. 3 with still other subdivisions.

FIG. 7 shows a vertical section through a Roots blower produced in conformity with the present invention.

In connection with the realization of the objects outlined above, it is necessary that a rotary piston be designed which as far as its entire profile contour is concerned may be defined by only two or a maximum of three circular arcs or segments identical for each rotary piston half, while the center points of said circular arcs or segments are located on the two main axes, possibly only on one of the two axes, namely on the longitudinal axis of the rotary piston profile. This consideration is based on the idea that principally theoretically there exists no rotary piston according to the Roots principle which as fair as its entire development is concerned corresponds to the just outlined requirements. This fundamental impossibility of a solution will best indicate the high level of the present invention. The profile of the Roots blade is nothing but a gear with two, three or four teeth which have to meet the primary requirement of gears, namely that the oppositely located tooth flank-s for purposes of a power transmission, in the present case for purposes of producing a seal, must establish contact in each rotatable angular position. With Roots blowers this requirement is made even more dilficullt in that in reality no contact must take place because such contact due to the lubricant-free movement would instantaneously cause damage. The imaginary contact point must actually be a very narrow limited tolerance because at said imaginary contact point the tooth flanks must be as close to each other as-possible without, however, contacting each other. Such closeness or tolerance should amount to fractions of tenth of a millimeter. In view of these gear technical conditions it is necessary in conformity with known laws that always only one tooth flank may be designed as at random curve, for instance a circular arc, whereas the counter flank cooperating with said first mentioned fiank must tollow a certain roller curve of a higher power, for instance the fifth power, which is dependent functionally on said first mentioned curve. It is for this reason that on shop drawings the blades are merely for drawing purposes or purposes of simplification indicated by similar circular arcs the center points of which, however, do not exclusively have to be located on the two main axes of symmetry of the rotary piston profile. At the same time it is prescribed that these thus indicated dimen sions are not valid for the manufacture inasmuch as these measurements can never yield the required precision. The manufacture proper has heretofore been eliected by the aid of sample templates which had to be produced by very highly skilled tool makers by a practical evolving of the blades and by tedious and very time consuming filing work. However, with the required high precision, even the best to 01 maker had to fail inasmuch as with the very narrow tolerances it could never be assured definitely where the igear tooth needed still further filing. These difiiculties are further increased by the fact that a precise measurement at these work pieces is impossible so that also in the so-called master template there will always be a certain non-controllable lack of precision. For the above reason it will be clear that heretofore in the entire construction of Roots blowers there was not available any ideal mass production for Roots blades adapted to be exchanged at random, and more or less post-machining or processing bad to be done during the installation of the blade profiles. It is for this reason that only such two Roots blades could be used in a housing which had been completely fitted to each other.

From the above it will also be clear that as generating flank an indefinite number of different curves may be selected and that also there exists an indefinite number of possibilities for shaping the Roots blades. In realizing the essence of the present invention, applicant started from a diametrically opposite problem. First, all rotary piston profiles composed of only two or a maximum of three circles identical to each rotary piston half and having their center points located on the two main axes of symmetry of the rotary piston profile were tested and examined as to their errors with regard to corresponding ideal tooth curves. in other words, contrary to the heretofore known procedure, according to which only ideal tooth curves were looked for, in the present case it was assumed from the very start that the examined blades are incorrect inasmuch as they are composed of the described circular arcs. It was then found that seen without dimensions there are practically only two solutions among an infinite number of profile shapes, which may be composed completely of circles having their center points located on the two main axes of the rotary piston profile so that with these the error limit is located within the required play between the Roots blades (due to heat tension and lubricant free running operations). When the center points of the limiting circular arcs are to be located on a main axis, namely the longitudinal axis of the rotary piston profile, there is actually only one solution. It is on the basis of this finding that according to the invention 2. Roots blade construction has become possible which, as far as its entire development is concerned, is confined by circular arcs the center points of which are located on the two or on one of the two main axes of the rotary piston profile. These axes are the axes of symmetry; the longitudinal axis of symmetry extending the long way of the rotary piston, and the transverse axis. being perpendicular thereto. More specifically, per each blade half there are only two or a maximum of three circular arcs selected so that the blades may be produced without special devices on customary lathes or grinding machines and that with the practically employed tolerance of 0.0015 of the rotary piston diameter such construction can be considered precise. As a result thereof, only one measuring operation and only one tolerance control is necessary so that in this way the basis is created on which a mass production of exchangeable Roots blades will become possible.

One of the shapes according to the present invention in which the central portion is designed concave does not have a visually noticeable corner in the profile contour as it is clearly shown in FIG. 2 of German Patent 415,864. Inasmuch as, as can be proved, there exists only this one form which meets the above mentioned requirements, the central portion of the Roots blade of German Patent 445,864 cannot be confined by a circle and consequently can also not be produced by drilling.

Referring now to the drawings in detail, the Roots blade profile shapes shown in FIGS. 1 and 3 represent shapes which with regard to their entire development can be described by two or a maximum of three circles having their centers located on the two main axes of the rotary piston profile without lack of precision with regard to customary tolerances. in FIGS. 2, 4, 5 and 6 illustrate how these profile shapes can practically be subdivided in order to be able to precisely machine all surfaces by customary machine tools without the necessity of resorting to special machines and devices. resent plane surfaces which, however, can also be very precisely produced or machine by planing or grinding with customary simple means. As has been illustrated, practically both Roots blade shapes are along separating lines 21, 41, 51 and 61 subdivided into three sections, namely the central or intermediate section 22, 42, 52 and 62 and the piston heads 23, 43, 53 and 63. As will be evident from the drawing, in each of the various modifications, most simple sections are obtained which can be measured precisely, a feature which was heretofore lacking with the Roots blower blades, and can be produced with any desired tolerance on customary machine tools, without requiring special devices so that it will now be possible to mass produce such Roots blower blades and easily to exchange such blades. In this connection it is important that the separating lines 21, 41, 51 and 61 lead into the merging points of the profile confining circles, i.e. where the profile confining circles engage each other, namely at 15, 25, 35, 45, 55 and 65. How the connection of the three rotary piston parts is eifected by standard means, whether the connection is elfected permanently by welding or detachably by screws or rivets or the like, will not affect the present invention and will be a mere matter of ordinary skill.

For the sake of completeness it may also be added that the now ascertained simple circular contouring of the rotary piston profiles according to the present invention, namely by circles having their center points located on the main axes of the rotary piston profile, could also permit other machining methods. If, for instance, the two rotary piston forms of FIGS. 1 and 3 were not subdivided into three sections as just described but were retained as single work piece, it would now also be possible to develop a machining tool, such as a cutter or the like, or a master template, for a desired copying machining process and such method would be very precise in view of the ascertained circular contouring. Thus, for instance, for machining the intermediate or central portion of the rotary piston according to FIG. 3, a cutter could be produced on a grinding machine with any desired precision, whereas heretofore an expensive and never precisely measurable, i.e. non-controllable profile cutter would have been necessary.

The subdivision of the rotary piston profile according to FIG. 2. could also be carried out in the manner shown in FIG. 4.

FIG. 7 shows a practical construction of a Roots blower in conformity with the present invention. erence numeral 66 designates the housing in which the blower pistons 77, 78 are contoured in conformity with FIG. 1 and rotate in the direction of the arrows a. The

drive of the pistons 77 and '78 may be elfected in cus V The embodiments shown Only the separating surfaces 41 in FIG. 4 rep:

-'Ihe ref- 6 tomary manner so that no further description thereof appears to be necessary. The housing opening 79 is in this instance the inlet and the opening 30 represents the outlet of the blower.

It is, of course, to be understood that the present invention is, by no means limited to the particular designs illustrated in the drawings but also comprises any modifications within the scope of the appended claims.

What I claim is:

1. A rotary piston for a Roots blower which rotates together with another piston of the same shape at the same speed, which comprises two substantially identical piston sections and an intermediate section interposed between and connected to said piston sections, the cross sectional contour of each of said two piston sections being defined solely by two unmodified circles having their center points located on the longitudinal axis of symmetry of said rotary piston, the contour of said intermediate section being defined solely by a circle having its center at the point of intersection of the axes of symmetry of said rotary piston, the deviation of said thus defined contour of said piston sections from the ideal tooth curve being within the tolerance of 0.0015 of the diameter of the rotary piston.

2. A rotary pistonfor a Roots blower which rotates together with another piston or the same shape at the same speed, which comprises two substantially identical piston sections and an intermediate section interposed between and connected to said piston sections, the cross sectional contour of each of said two piston sections being defined solely by two unmodified circles having their center points located on the longitudinal axis of symmetry of said rotary piston, the contour of said intermediate section being defined solely by a circle having its center at the point of intersection of the axes of symmetry of said rotary piston, the deviation of said thus defined contour of said piston sections from the ideal tooth curve being within the tolerance of 0.0015 of the piston diameter, one of the defining circles for each piston section having its center at the point of intersection of the axes of symmetry of the rotary piston.

3. A rotary piston according to claim 2 in which said intermediate section is a member separate from said piston sections and is connected to said piston sections along surfaces passing through the points of intersection of the outer contour of said piston sections with the outer contour of said intermediate section.

4. A rotary piston according to claim 3, in which the surfaces along which said intermediate section and said piston sections are connected are planar.

References Cited in the file of this patent UNITED STATES PATENTS Re. 2,369 Roots Oct. 2, 1866 685,775 Lindsay Nov. 5, 1901 2,451,603 Barker Oct. 19, 1948 FOREIGN PATENTS 415,864 Germany July 4, 1925 282,752 Great Britain May 31, 1928 639,992 Germany Dec. 18, 1936 645,402 France June 27, 1928 889,092 France Sept. 20, 1943 953,289 Germany Nov. 29, 1956 1,170,967 France Sept. 28, 19 58 

1. A ROTARY PISTON FOR A ROOTS BLOWER WHICH ROTATES TOGETHER WITH ANOTHER PISTON OF THE SAME SHAPE AT THE SAME SPEES, WHICH COMPRISES TWO SUBSTANTIALLY IDENTICAL PISTON SECTIONS AND AN INTERMEDIATE SECTION INTERPOSED BETWEEN AND CONNECTED TO SAID PISTON SECTIONS, THE CROSS SECTIONAL CONTOUR OF EACH OF SAID TWO PISTON SECTIONS BEING DEFINED SOLELY BY TWO UNMODIFIED CIRCLES HAVING THEIR CENTER POINTS LOCATED ON THE LONGITUDINAL AXIS OF SYMMETRY OF SAID ROTARY PISTON, THE CONTOUR OF SAID INTERMEDIATE SECTION BEING DEFINED SOLELY BY A CIRCLE HAVING ITS CENTER AT THE POINT OF INTERSECTION OF THE AXES OF SYMMETRY OF SAID ROTARY PISTON, THE DEVIATION OF SAID THUS DEFINED CONTOUR OF SAID PISTON SECTIONS FROM THE IDEAL TOOTH CURVE BEING WITHIN THE TOLERANCE OF 0.0015 OF THE DIAMETER OF THE ROTARY PISTON. 